Oracle APEX Pass null to REST Data source on POST/PUT - json

Question

I would like to know if there is a way to pass a null on a json with REST Data Source? Currently, the problem i have is the null have double quote "null"
Here is my Request Body Template
{"dt_deb":"#DT_DEB#","dt_fin":"#DT_FIN#", "nom_proj":"#NOM_PROJ#","id_doss_clien":"#ID_DOSS_CLIEN#"}
In my Operation Operator i've put default value to null
And this is my PL/SQL i use to call my PUT REST API
declare
l_parameters apex_exec.t_parameters;
l_date date;
begin
if :REQUEST = 'CHANGE' then
apex_exec.add_parameter( p_parameters => l_parameters, p_name => 'id', p_value => :P910_ID_PROJ_COLAB_TRNFO_NUM );
end if;
apex_exec.add_parameter( p_parameters => l_parameters, p_name => 'ID_PROJ_COLAB_TRNFO_NUM', p_value => OTN.otnk_api_gestn_seq.nuf_otnprj );
apex_exec.add_parameter( p_parameters => l_parameters, p_name => 'NOM_PROJ', p_value => :P910_NOM_PROJET );
apex_exec.add_parameter( p_parameters => l_parameters, p_name => 'DT_DEB', p_value => to_char( to_date( SYSDATE ), 'YYYY-MM-DD' ) || 'T00:00:00Z' );
IF :P910_DT_FIN != 'O' THEN
apex_exec.add_parameter( p_parameters => l_parameters, p_name => 'DT_FIN', p_value => to_char( to_date( SYSDATE ), 'YYYY-MM-DD' ) || 'T00:00:00Z' );
END IF;
apex_exec.add_parameter( p_parameters => l_parameters, p_name => 'ID_DOSS_CLIEN', p_value => :P910_ID_DOSS_CLIEN );
apex_exec.execute_rest_source(
p_static_id => 'DETAIL_PROJET',
p_operation => case :REQUEST
when 'CHANGE' then 'PUT'
when 'CREATE' then 'POST' end,
p_parameters => l_parameters );
end;
Like you see, DT_FIN are optional and can have null, but because i've put null as defaut parameter, when i check the APEX Debug tool, i see "null"
Any idea how i can manage null value with a date?
Thanks for your help.